Musnad Ahmad 25:1672

Hujayn ibn al-Muthanna narrated to us, Isra'il narrated to us, from Abu Ishaq, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "The Prophet, peace and blessings of Allah be upon him, used to perform Witr with three [rak'ahs]: with 'Glorify the name of your Lord, the Most High' (Surah Al-A'la), and 'Say, O disbelievers' (Surah Al-Kafirun), and 'Say, He is Allah, [who is] One' (Surah Al-Ikhlas)."